PROJECT SUMMARY/ABSTRACT Patients with single-sided deafness (SSD) report significant difficulty understanding speech and localizing sound, especially when the sounds originate on the deaf side. Recently, some SSD patients have received a cochlear implant (CI) in their deaf ear. Current fitting strategies aim to maximize speech perception through the CI alone by allocating all spectral information across the electrode array without regard to tonotopic placement of each electrode along the basilar membrane. In this study, we propose a new fitting philosophy for patients with one acoustic-hearing ear, whereby CI maps are designed to best complement information from the normal-hearing (NH) ear. In Aim 1, we investigate the mechanisms of binaural temporal-envelope beat sensitivity in NH listeners using bandpass filtered pulse trains with parameters including stimulus level, filter bandwidth, filter slope, and spectral overlap using bandpass filtered pulse trains. We find the minimum baseline ITD and spectral mismatch that NH listeners can tolerate while maintaining their ability to detect ITDs. New and experienced SSD+CI listeners will complete similar tasks and the results will be used to adjust their CI maps in an effort to reduce the frequency-mismatch between the NH and CI ear. In Aim 2, we will measure sound localization and speech understanding in noise to compare performance with the standard clinical map and the new experimental map based on a closer between-ear spectral match. Performance will be measured over time to ensure sufficient exposure and allow time for adjustment. In the localization task, we present listeners with stimuli designed to separate the relative contributions of interaural level differences and interaural time differences in SSD+CI patients. In the first speech-in-noise experiment, we will measure speech recognition performance in three spatial configurations using two masker types: speech-shaped noise and two- talker babble. By employing both energetic (noise) and informational (two-talker babble) maskers, we will test the hypothesis that improvement in localization produced by a frequency-to-place maps are superior to standard clinical maps especially in cases where the binaural benefit goes beyond ?better ear? performance to encompass complex listening situations where informational masking dominates. The outcomes of this study will provide new information on binaural interactions in SSD patients with a CI, and may provide a new and more successful approach to fitting a CI in cases where considerable residual hearing exists in the contralateral ear.